Many non-invasive or minimal-invasive therapies (such as MR-guided high intensity focused ultrasound (HIFU) or radio frequency (RF) deep hyperthermia) require accurate temperature monitoring in the human body. MR-based temperature mapping in tissue is performed using the temperature-dependent proton resonance frequency shift (PRFS) phenomenon, the diffusion coefficient (D), the longitudinal (T1) and transversal (T2) relaxation times, proton density (PD), magnetization transfer, as well as temperature sensitive contrast agents. In particular, the PRFS method is commonly used for MR thermometry (MRT) because it is a simple and robust MRT method in water-based tissues. T1 based thermal mapping in combination with a variable flip angle (VFA) method is used to track temperature changes in adipose tissue. Relative temperature changes in tissue and fat can also be measured by leveraging the temperature-dependent PD signal.
Some of the thermal therapies have to be performed very close to bony structures; or energy used for the treatment has to be transmitted through bone. The potential risk of bone heating makes it highly desirable to further acquire temperature updates in bone for accurate treatment monitoring, which is not clinically performed yet. Cortical bone, as well as other bony structures, have ultra-short T2 relaxation properties (i.e. T2<1 milliseconds), which requires dedicated acquisition techniques to capture the rapidly decaying MR signals with appropriate signal-to-noise ratio (SNR). Examples of existing methods that perform MRT in or around bone include: a dual-echo single slice two dimensional (2D) spoiled gradient echo sequence for simultaneous tracking of temperature change in bone and cerebral tissue; the normalized change of signal magnitude of the short echo time images is used to extract the temperature change of bone. The long-echo time images provide phase information and temperature changes are extracted based on the PRFS method for soft tissue. Nevertheless, high SNR is utilized, whereas with this method, only 20% of SNR could be achieved; a three dimensional (3D) ultra-short echo time (UTE) sequence to evaluate T1 changes due to heating in cortical bone and T2 changes in yellow bone marrow; UTE imaging to compensate for phase-aberrations in the skull; combined UTE with spectroscopic imaging (UTESI) to monitor temperature changes in the musculoskeletal system; and qualitative and quantitative results of UTE-based MRT of cortical bone.
Recently, zero echo time (ZTE) MR bone imaging in the head has been introduced as a technology in multi-modality diagnostic imaging. The echo time (TE) of an MRI sequence is considered zero when the generation of the transverse magnetization coincides with the acquisition of the k-space center. This feature is characteristic for ZTE techniques with the 3D radial readout gradient active during excitation. For example, PD weighted images are acquired using e.g. the rotating ultra-fast imaging sequence (RUFIS) to extract bone structures used for image segmentation. ZTE features a non-selective hard pulse excitation together with 3D center-out radial sampling with the spokes arranged on a spiral path requiring minimal gradient ramping in between repetitions. The PD weighting is achieved by using very small flip angles. Applying a variable flip angle method, ZTE allows for an efficient T1 mapping, too. A limitation of using the temperature-dependent PD signal to measure temperature changes in tissue, where PD weighting is imperfect or has to be compromised, is the T1 signal dependency influencing the PD weighted signal resulting in signal contrast changes and hence incorrect temperature map calculations. Perfect PD weighting is achieved in the limiting case of a flip angle (FA)→0 and TR→infinity. Hence, this limitation could be overcome by increasing the repetition time (TR), which prolongs the total acquisition time to a point that makes the PD weighted temperature mapping impossible for applications that need a temperature update every few seconds.
The present method will address the issues as detailed above. The method will desirably provide a technique for MRT to measure temperature changes in MR-visible tissues based on PD and T1 weighted ZTE for accurate treatment monitoring.